Sensors for detecting the proximity of metallic objects are known. Typically, such a proximity sensor operates by transmitting a magnetic field from a coil (or other antenna) on the sensor toward the metallic object. The transmitted magnetic field in turn stimulates eddy currents in the metallic object. As the magnetic flux lines reach the object's surface, the flux lines are reflected due to the large change in conductivity (almost infinite) between air and the conducting metal. The reflected magnetic flux reduces the total flux at the coil causing either the voltage on the coil to be reduced (in the case of a constant current coil drive) or the coil current to increase (in the case of a constant voltage drive).
As the number of flux lines that are reflected by the target increases, the coil voltage is reduced (or coil current is increased) more and more. Thus, as the sensor and metallic object come closer to one another, the voltage at the coil decreases (or the current in the coil increases), thereby providing an indication of increasing proximity. Likewise, as the sensor and metallic object become farther apart from one another, the voltage at the coil increases (or the current in the coil decreases), thereby providing an indication of decreasing proximity.
Despite the fact that such proximity sensors are known in the art, conventional proximity sensors are limited in terms of their range of operation. As the distance of a target metallic object increases, the proximity sensor's sensitivity to changes in the object's position decreases significantly. Although amplification of the proximity sensor's output signal can yield some improvement in the proximity sensor's sensitivity at larger distances, such amplification does not satisfactorily improve the performance of the proximity sensor. In particular, amplification ceases to improve the proximity sensor's performance when the sensitivity of the proximity sensor to changes in the target object's distance becomes so minimal as to be indistinguishable from noise or signal draft.
Standards for the performance of proximity sensors for metallic objects are set forth in the IEC document entitled “Low Voltage Switchgear and Control Gear, Part 5, Section 2 “Proximity Switches”, (CEI/IEC 947-5-2).” The document specifies an operating distance for various diameter sensors as noted below in Table One.
TABLE ONEIEC Required Detection DistancesSensor Type andEmbedded SensorNot Embedded SensorDiameterRangeRangeA8mm 1 mm 2 mmA12mm 2 mm 4 mmA18mm 5 mm 8 mmA30mm10 mm15 mmB4mm .8 mm —B6mm 1 mm—C26mm10 mm15 mmC30mm10 mm15 mmC40mm15 mm20 mmD60mm25 mm—D80mm40 mm—
Due to the limitations of conventional proximity sensors as discussed above, conventional proximity sensors are typically able to only exceed the above standards by a factor of 2 to 3. Nevertheless, it would be advantageous if new proximity sensors could be developed that were significantly more sensitive than extended range conventional sensors. In particular, it would be advantageous if such new proximity sensors could successfully sense metallic objects at distances that were an order of magnitude (e.g., 10 times) greater than those specified by the IEC. Further, it would be advantageous if such new proximity sensors could be easily and cost-effectively implemented.